animal is in good condition at the outset, it has been found that 

 usually only about 10 to 15 per cent, of the energy required for 

 the vital activities during fasting is derived frofm protein, the re- 

 mainder being drawn from the body fat. If, however, the body 

 was originally deficient in fat, or when it becomes so as a result 

 of fasting, the protein is used more freely, so that when the body 

 becomes practically exhausted of fat almost the entire supply of 

 energy may be derived from the breaking down of protein. In other 

 words, in the absence of the special fuel material (fat) in the body, 

 its working machinery begins to be used and this naturally leads 

 to a speedy death. On the other hand, some breaking down of 

 protein always takes place however much fat the body may con- 

 tain. This unavoidable loss of protein in fasting may be roughly 

 compared to the wear of the working parts of a machine. Finally, 

 there is likewise a continual loss of ash ingredients as well as of 

 protein from the organs of the fasting animal. 



The necessary operations of the vital machinery of the fasting 

 animal, then, require : 



First, certain rather small amounts of protein and ash. 



Second, a certain supply of energy, of which the greater part is 

 usually supplied by the breaking down of fat, but to which the body 

 protein contributes to a greater or less extent. 



Experiments like those described on subsequent pages, in which 

 the amounts of protein and energy required for the vital functions 

 of the fasting animal are determined, either directly or indirectly, 

 give what may be called the maintenance requirements', that is, 

 _ they show the minimum amounts necessarily expended for the mere 

 continuance of life. The maintenance requirements in this sense 

 are to be distinguished from the maintenance ration, i. e., from the 

 amount of food of any given kind which must be supplied to the 

 animal in order to satisfy these requirements and so prevent a loss 

 of protein, ash or fat from the body. 



THE DETERMINATION OF THE MAINTENANCE REQUIREMENTS. 



With animals such as man, carnivora or swine, having a com- 

 paratively simple digestive apparatus and living on relatively con- 

 centrated food, the maintenance requirements in the foregoing 

 sense can be determined without special difficulty in a relatively 

 short period of fasting, although an exact experiment involves the 



of a respiration apparatus or respiration calorimeter. The total 



^"se 



